Josie's height = 5 ft 6 in. = 5 1/2 ft = 11/2 ft.
Josie's shadow = 6 ft.
Ratio of shadow to height = 6 / 11/2 = 12/11
So, It would be equal to ratio of building's shadow to building's height.
18/x = 12/11
x = 18*11 /12
x = 3*11 / 2
x = 16.5
In short, Your Answer would be 16.5 Feet
Hope this helps!
